I'm trying to add a user login there but since I'm new to CSS and learning as I go I would really really appreciate a quick guidance in this specific use case. I learn as I do. :P

Please see the source code of my site to see that everything is very structured and not complicated to add new things to; it's just that I have literally no idea where to start to add a Div to that specific location.

You'll probably want to float it in your header in the right upper corner.
This can be easily done by placing the div in your <div id="header"> and float it right.
(Same way as you've floated your logo to the left)